INTRODUCTION
The eruption of Mount St. Helens, Washington, on May 18, 1980, caused concern that this activity might signal the start of eruptive activity of other volcanoes in the Cascade Range. During the Holocene Epoch (last 10,000 years), eruptive activity at Mount Shasta in northern California has occurred on the average of about once every 800 years (Miller, 1980)-a frequency roughly equivalent to that of Mount St. Helens (Mullineaux and Crandell, 1981). Phenomena during eruptions that could endanger people or property in the Mount Shasta area include debris flows triggered by rapid runoff of melting snow and ice from pyroclastic flows, pyroclastic surges, lava flows, and tephra. Debris flows of noneruptive origin also may cause hazards to life and property along downstream reaches of channels if the capacity of streams is reduced by debris-flow deposits and overbank flooding occurs.
Mount Shasta is a large stratovolcano in northern California about 61 km south of the Oregon-California border (figs. 1 and 2 below). Of the volcanoes in the Cascade Range, Mount Shasta is the second highest (altitude, 4,316.6 m) and one of the largest. The average basal diameter of Mount Shasta is about 27 km. Williams (1934) indicated that the Mount Shasta cone is perhaps larger in volume and height than Mount Rainier, Washington, because the cone of Mount Rainier rises above an elevated platform of older formations at an altitude of 2,440 m.
Debris flows and floods related to eruptive activity as described by Crandell (1971) are caused by a number of mechanisms. Debris flows may occur after the water contained in a volcanic crater is ejected during an eruption and spilled on the slopes. Debris-flow material also may be extruded directly by volcanoes. Avalanches of rock debris may cause debris flows by temporarily damming streams. Eventually, water spilling over the dam causes erosion, and the deposits are swept downstream as debris flow. Avalanches of hot rock debris have caused debris flows by melting snow and ice. Eruptive explosions may cause landslides of rocks weakened by hydrothermal alteration. As these landslides descend the volcano, the snow and glaciers are melted and debris flows are created. Debris flows were apparently created by rapid snowmelt near the summit of Lassen Peak during a 1915 eruption (Marron and Laudon, 1987). On Mount Shasta, debris flows related to eruptions have not occurred since at least 1786 (Williams, 1934).
Some flows described as debris flows on Mount Shasta may have been streamflows with extremely high concentrations of suspended sediment. These flows, defined as hyperconcentrated streamflow (Beverage and Culbertson, 1964), contain water with suspended-sediment concentrations between 40 and 80 percent by weight. When coarser particles are freely suspended in a mixture of water and fine particles, the flow is a hyperconcentrated fluid, in which the sediment-water mixture has no shear strength and acts as a typical (Newtonian) fluid. The conditions associated with the change from debris flow to hyperconcentrated flow vary, depending on the particle size and sediment concentration of the slurry (Pierson and Scott, 1987). Sedimentologic data for Mount Shasta were generally insufficient to distinguish hyperconcentrated flow from debris flows. In this report, the term "debris flow" is used to describe all types of flows (debris flows, mudflows, and hyperconcentrated streamflow) not re lated to eruptive activity.
A debris-flow deposit is the sediment or rock mass layer that remains after cessation of flow. A debris fan develops when debris-flow deposits become repeatedly stacked and spread over an extensive area. These fans, such as those found on Mud Creek at Road 13 crossing (fig. 3 below), are typical of the alluvial fans on the flanks of Mount Shasta. Debris fans commonly are seen below 1,800-m altitude, which is about 10,000 m from the summit. These debris fans generally are incised between 1,200- and 1,800-m altitude; exceptions are Ash, Brewer, and Gravel Creeks, which have extensive debris-flow deposits with minor incision between 1,700- and 1,900-m altitude.
In response to the increased awareness of flood potential of streams on the flanks of volcanoes, this report was prepared with the purpose of describing the sources, potential, and characteristics of debris flows of noneruptive origin on Mount Shasta. This report presents an evaluation of data collected before and during the study and describes significant characteristics of debris flows. Attention was restricted to "cold" flows; that is, to flows not directly related to eruptive activity. Debris flows were evaluated on the basis of inferred origin, stream location, thickness, areal extent of inundation, and estimated velocity. Procedures used to estimate the probability of future debris flows and to identify potentially hazardous are as included geomorphic, climatic, and dendrochronologic techniques (Hupp and others, 1987). The definition of areas of debris-flow deposition and the present condition of channels on the flanks of the mountain were estimated using hydraulic analyses, topographic surveys, and aerial photographs. Flow depth and areas of inundation of various debris flows were estimated from studies and observations of historical debris flows and the resulting deposits.
The characteristics and hydrology of debris-flow activity and associated hazards of Mount Shasta are described by Osterkamp and others (1986), Hupp and others (1987), and Blodgett and others (1988). Debris-flow activity and glaciers on Mount Shasta are discussed by Hill and Egenhoff (1976), Hill (1977), Miller (1980), Hupp (1984), and Rhodes (1987).